Carbon nanotubes (CNTs) have intriguing physical and chemical properties which have consequently made them the object of numerous ongoing studies (Ajayan et al., Top. Appl. Phys., vol. 80, p. 391, 2001; Dai, Acc. Chem. Res., vol. 35, p. 1035, 2002). As a result of some of these studies, carbon nanotubes have been found to be excellent cathode materials for field emission displays because of their high aspect ratio and outstanding chemical inertness (U.S. Pat. No. 5,773,921). Single-wall carbon nanotubes (SWNTs) are hollow carbon fullerene tubes that have diameters from 5 angstroms to several nanometers (nm) and can be microns (μm) long or longer. Multi-wall carbon nanotubes (MWNTs) are similar, but comprise more than one concentric layer of carbon forming the tube. It has been suggested that aligned carbon nanotubes may have good field emission properties because they have higher geometric field enhancement (Wang et al., Appl. Phys. Lett, vol. 72, p. 2912, 1998). CNTs can be produced by chemical vapor deposition (CVD) (Nikolaev et al., Chem. Phys. Lett., vol. 313, p. 91, 1999; Huang et al., Appl. Phys. A, vol. 74, p. 387, 2002), arc discharge (Journet et al., Nature, vol. 388, p. 756, 1997), laser ablation (Thess et al., Science, vol. 273, p. 483, 1997), and other techniques (e.g., Derycke et al., Nano Letters, vol. 2 (10), p. 1043, 2002). Additionally, vertically-aligned CNTs can be grown on substrates possessing nanoscale metal catalysts using CVD methods (Huang et al., 2002) at temperatures from about 550° C. to about 1200° C.
All of the abovementioned techniques, however, have poor growth uniformity and none can practically deposit carbon nanotubes over large areas. Furthermore, the growth conditions require relatively high temperatures, which impede their utilization with low-temperature and generally inexpensive substrate materials.
Another problem with using the abovementioned CNT growth techniques for generating the cathode material for field emission displays is that the density of the CNTs produced may be too high. Researchers have found evidence that the field emission properties of high density CNT cathodes is less than expected because the neighboring nanotubes shield the extracted electric fields from each other (Bonard et al., Advanced Materials, vol. 13, p. 184, 2001). As a result, high-resolution lithography has been employed to control CNT density by creating catalytic dots capable of growing CNTs (Huang et al., Appl. Phys. A, vol. 74, p. 387, 2002). This method is very expensive, however, and requires growth on high-temperature substrates.
Thus, there is a demonstrated need to be able to harvest fabricated CNTs and apply or dispense them onto various substrate materials at low temperatures. There is also a need to be able to control the density of the CNTs in an effort to optimize their field emission properties.